Combustors are commonly used in industrial and power generation operations to ignite fuel to produce combustion gases having a high temperature and pressure. Various competing considerations influence the design and operation of combustors. For example, higher combustion gas temperatures generally improve the thermodynamic efficiency of the combustor. However, higher combustion gas temperatures also promote flashback or flame holding conditions in which the combustion flame migrates towards the fuel being supplied by nozzles, possibly causing severe damage to the nozzles in a relatively short amount of time. In addition, higher combustion gas temperatures generally increase the disassociation rate of diatomic nitrogen, increasing the production of nitrogen oxides (NOX). Conversely, lower combustion gas temperatures associated with reduced fuel flow and/or part load operation (turndown) generally reduce the chemical reaction rates of the combustion gases, increasing the production of carbon monoxide and unburned hydrocarbons.
In a particular combustor design, an end cover or breach end connected to a combustor casing may define a combustor head end, and a cap assembly that extends radially across a portion of the combustor may separate the head end from a combustion chamber. One or more fuel nozzles connected to the breech end in a cantilevered fashion may extend downstream from the breech end to the cap assembly. The fuel nozzles may be radially arranged in the combustor head end to mix fuel with a working fluid prior to combustion in the combustion chamber.
Increasing an axial length and/or volume of the head end allows more time for the fuel and compressed working fluid to mix prior to combustion. The enhanced mixing allows leaner combustion at higher operating temperatures to protect against flashback or flame holding while also controlling undesirable emissions. However, increasing the axial length and/or volume of the head end may lead to harmful combustion dynamics that reduce the useful life of one or more combustor components. For example, increasing the axial length of the head end may result in lower natural frequencies associated with the cantilevered fuel nozzles, leading to high cycle fatigue failure of the fuel nozzles and downstream components. Alternately, or in addition, the combustion dynamics may produce pressure pulses inside the fuel nozzles and/or combustion chamber that affect the stability of the combustion flame, reduce the design margins for flashback or flame holding, and/or increase undesirable emissions. Therefore, an improved system and method for supporting fuel nozzles inside a combustor that increases the natural or resonant frequencies created by the fuel nozzles, improves the high cycle fatigue limits, and/or reduces undesirable combustor dynamics would be useful.